ci: remove Docker Hub login, use REGISTRY_TOKEN for Gitea auth

Fixes CAR-1009.

- Remove Docker Hub login step (not needed)
- Rename 'Log in to GHCR' → 'Log in to Gitea Container Registry'
- Use REGISTRY_TOKEN instead of GITEA_TOKEN for Gitea auth

Co-Authored-By: Paperclip <noreply@paperclip.ing>

.gitea/workflows/ci.yml

@@ -99,20 +99,13 @@ jobs:
           echo "version=$VERSION" >> "$GITHUB_OUTPUT"
           echo "CalVer tag: $VERSION"
 
-      - name: Log in to Docker Hub
-        if: github.event_name == 'push'
-        uses: docker/login-action@v3
-        with:
-          username: ${{ secrets.DOCKERHUB_USERNAME }}
-          password: ${{ secrets.DOCKERHUB_TOKEN }}
-
       - name: Log in to Gitea Container Registry
         if: github.event_name == 'push'
         uses: docker/login-action@v3
         with:
           registry: ${{ env.REGISTRY }}
           username: ${{ github.actor }}
-          password: ${{ secrets.GITEA_TOKEN }}
+          password: ${{ secrets.REGISTRY_TOKEN }}
 
       - name: Extract metadata
         id: meta